1. Field of the Invention
The invention relates to steering systems suitable for use in off-highway applications and, more particularly, relates to a steering system usable, e.g., in a watercraft such as a boat or a jet ski or in an off-highway wheeled vehicle such as a tractor or other agricultural implement. Specifically, the invention relates to a steering system that incorporates (1) a manually selectable multi-ratio speed changer that permits the responsiveness or required steering effort of the system to be varied upon demand and/or (2) incorporates a torque gate acting as an anti-backdrive mechanism that prevents forces imposed on or by the steered mechanism from being transmitted back through the steering system to the steering mechanism.
2. Discussion of the Related Art
A wide variety of steering systems are available for transmitting steering forces from a steering mechanism such as a steering wheel to a steered mechanism such as a steered wheel or a rudder. The optimal steering response characteristics for such steering systems vary depending on prevailing operating conditions. Forces imposed on or by the rudder or other steered mechanism may also be transmitted back through the steering system to the steering wheel or other steering mechanism. These forces can result in wear and tear on steering system components and risk operator discomfort or even loss of vehicle control. Applications include both off-highway wheeled vehicles and watercraft.
Wheeled vehicles exhibiting the above noted characteristics include, but are not limited to, agricultural machines such as tractors. Vehicles of this type typically incorporate a steering wheel as the steering mechanism and one or more steered wheels as the steered mechanism. Steering forces are transferred to the steered wheel by a torque transfer system that includes a gear reducer that reduces the rotational velocity of a rotary drive input element for the steered wheel (such as a drive pinion for a rack and pinion steering system) relative to the rotational velocity of the steering wheel. Responsiveness and steering forces in these systems are necessarily inversely related. A system having a relatively high numerical speed reduction ratio can be operated with relatively low steering forces but requires a relatively large range of angular movement of the steering wheel to effect a given vehicle turning angle. Conversely, a system with a lower speed reduction ratio requires more operator effort to effect a given turning angle but requires a smaller range of steering wheel angular motion to effect a given turning angle. The reduction ratios of these systems are usually fixed. Steering response characteristics and steering effort are invariable as a result. This lack of versatility hinders optimal steering over the full range of operating conditions typically experienced by many vehicles.
For instance, in the case of a tractor, a relatively high speed reduction ratio (e.g., in the order of 6:1) might be desirable for in-field operating conditions in which the tractor is operated over rough terrain and under conditions in which the steering wheel is relatively difficult to turn but in which sharp turns are unnecessary. On the other hand, a relatively low speed reduction ratio (e.g. on the order of 3:1) might be desirable when the tractor is operating in smoother fields or when sharp turns are required at the ends of rows. The reduction ratios of most known steering systems are set either to provide a compromise between these (and possibly other) desired response characteristics or are simply set to provide the response characteristic most often exhibited by the vehicle. Neither arrangement is optimal.
In watercrafts such as boats, jet skis, etc., a steered mechanism such as a pivoting outboard motor, jet nozzle, or a separate rudder is steered by operating a steering mechanism, typically a steering wheel or handlebars. In the case of boats having steered outboard motors, the steering forces are typically transmitted to a pivoting mount for the motor by either a mechanical or hydraulic system. Mechanical systems typically have relatively low speed reduction ratios, resulting in good responsiveness but requiring relatively high steering effort. Hydraulic systems typically have relatively high speed reduction ratios, resulting in relatively low responsiveness but requiring relatively little steering effort. They are also more expensive than mechanical systems, are prone to leaks, and are heavy. Nearly all such systems, like the corresponding steering systems for land-based vehicles, have a set, invariable speed reduction ratio. This limitation is just as problematic, if not more problematic, for marine applications as for land-based applications. Specifically, a relatively high speed reduction ratio would be desirable when operating a boat at high speeds in open water because sharp turns are seldom required under these conditions, but reaction forces on the rudder impose substantial resistance to turning. Conversely, when narrow turns are required at low speed operation but against lower resistance to turning, as would normally occur when maneuvering around a dock area, the operator would prefer to have more responsiveness, making a relatively low gear reduction ratio desirable. The problems encountered at lower speeds are compounded by the fact that the rudder is typically less effective at changing the heading of a boat at slower speeds, hence requiring a greater angular change on the rudder to achieve the same steering effect. Known mechanical steering systems, typically having relatively low speed reduction ratios (on the order of 3:1), exhibit adequate response characteristics at lower speeds but are relatively hard to steer at high speeds in the open water. Hydraulic systems, typically having a gear reduction ratio on the order of 6xc2xd:1, are relatively easy to steer at all speeds but provide less responsiveness than some operators desire at low speeds during sharp turns.
Some proposals have been made to vary the responsiveness of a steering system to render the system more versatile. For instance, U.S. Pat. No. 5,018,469 to Carlson proposes a variable ratio steering helm for a boat. Steering forces are transmitted to a rudder via a rack and pinion system coupled to a steering wheel. The pinion of the system is mounted eccentrically relative to the gear that drives it. This eccentricity is set to provide progressively increased leverage as the steering wheel progressively turns, thereby permitting the system to counteract increasing rudder forces encountered in sharp turns. The system disclosed in the Carlson patent is less than perfect because the speed reduction ratio provided by the system varies automatically, leaving the operator without a sense of control. The speed reduction ratio also cannot be varied for any particular steering wheel position. The operator therefore cannot tailor the response characteristics to meet the needs of the prevailing operating conditions.
U.S. Pat. No. 3,225,620 to Dubin discloses a multiple ratio steering system for a boat that permits an operator to select one of two distinct speed reduction ratios. The system including a steering shaft coupled to a steering wheel, an output shaft coupled to the rudder, and a system of gears disposed between the steering shaft and a pinion coupled to the boat""s rudder. These gears include first and second spur gears coupled to the steering shaft and third and fourth spur gears mounted on an output shaft that also bears the pinion. The first and second spur gears are slidable along the steering shaft by way of a key connected to an operator-manipulated adjustment rod. Depending upon the shifting position selected by the operator, either the first and third gears or the second and third gears mesh. One speed reduction ratio is obtained when the first and third gears mesh and a second, different speed reduction ratio is obtained when the second and fourth gears mesh. While this system provides operator control over the reduction ratio of a steering system, it is difficult to implement because the gears that transfer torque through the steering system must translate axially relative to the system. System robustness suffers as a result. The system must also be relatively large to accommodate the sliding movement of the two axially-spaced spur gears, prohibiting its use in many applications.
Both land-based and marine steering systems are also potentially subject to undesirable backdrive to the steering mechanism from the steered mechanism. In the case of marine applications, reaction forces on the rudder, steering torques, and other forces imposed on or by the rudder can be transmitted back through the steering system to the steering wheel. The operator must vigilantly maintain control of the steering wheel in order to prevent these transmitted forces from diverting the boat from its intended path. The vibrations and shocks transmitted by these backdrive forces also subject the entire steering system to substantial wear and tear. Land-based vehicles, particularly tractors or other vehicles that must travel over rough or uneven terrain, exhibit similar problems.
Proposals have been made to reduce or eliminate backdrive forces to a steering mechanism from a steered mechanism through the incorporation of an anti-feedback mechanism into the steering system. Examples of such systems are disclosed in U.S. Pat. No. 2,819,777 to Kosch and U.S Pat. No. 2,947,278 to Magill. Both systems employ a spring that is located between rotary input and output elements of the steering system. The spring is operable to 1) grip a stationary surface upon rotation of the output element relative to the input element to prevent backdrive and 2) release from the stationary surface upon rotation of the input element relative to the output element so as to permit steering forces to be transmitted to the steered mechanism. These structures, however, are relatively complex and are not sufficiently robust for use in many applications. They also are not designed for use with a multi-ratio speed changer or to be placed in the same package as such a speed changer.
The need has therefore arisen to provide a steering system that is usable, e.g., in marine applications or off-highway land vehicle applications and that incorporates measures to permit the operator to select between multiple, distinct speed change ratios.
The need further exists for a steering system having a multi-ratio speed changer that is robust, compact, and relatively simple.
The need further exists to provide a steering system having a simple, compact, and robust anti-backdrive mechanism usable either in conjunction with or independently of a multi-ratio speed changer.
In accordance with a first aspect of the invention, a steering system comprises a manually operated steering mechanism, a steered mechanism, and a steering transmission coupling the steering mechanism to the steered mechanism. The steering transmission includes a multi-ratio speed changer and a torque gate. The speed changer has an input element that is coupled to the steering mechanism, an output element that transmits steering forces to the steered mechanism, and a shifter that is movable between at least first and second shifted positions to vary a speed change ratio of the speed changer between at least first and second distinct change ratio. The torque gate couples the output element of the speed changer to the steered mechanism so as to permit steering forces to be transmitted to the steered mechanism from the speed changer in either direction while preventing backdrive forces from being transmitted to the speed changer and ultimately to the steering wheel from the steered mechanism.
Preferably the speed changer comprises at least one planetary gear set including a planet gear, a ring gear, and a plurality of planet gears positioned between the ring gear and the sun gear. At least one of the gears is selectively fixable from rotation upon movement of the shifter into one of the shifted positions thereof in order to alter the change ratio of the speed changer. Even more preferably, the speed changer includes at least two planetary gear sets, each of which provides a different speed change ratio when selected. Movement of the shifter into a selected one of the shifted positions thereof selects one of the planetary gear sets for the transfer of torque from the steering mechanism to the steered mechanism. In a particularly preferred embodiment, the first planetary gear set has a first sun gear, a first ring gear, and a first plurality of planet gears disposed between the first ring gear and the first sun gear. The second planetary gear set has a second sun gear, a second ring gear, and a second plurality of planet gears disposed between the second ring gear and the second sun gear. The first and second sun gears typically are of first and second different diameters. Movement of the shifter into one of the shifted positions thereof arrests at least one gear of a corresponding one of the first and second planetary gear sets from rotation.
The torque gate preferably comprises a stationary surface, a release driver, a locking driver, and a wrapped spring. The release driver is coupled to the output element of the speed changer and is rotatable relative to the stationary surface. The locking driver is coupled to the steered mechanism and is rotatable relative to the stationary surface. The release driver and the locking driver have axially-extending drive surfaces that face one another with a circumferential gap therebetween. The wrapped spring that has at least one coil that is disposed adjacent the stationary surface. The release, driver, the locking driver, and the stationary surface are dimensioned and configured relative to one another such that, (1) upon rotation of the release driver in either direction under torsional forces imposed on the torque gate by the speed changer, the spring rotates relative to the stationary surface so as permit the drive surface of the release driver to engage the drive surface on the locking driver and to drive the locking driver to rotate, thereby permitting torque transfer to the steered mechanism from the output element of the speed changer, and (2), in the absence of the imposition of an overpowering steering force on the release driver from the speed changer, and upon rotation of the locking driver in either direction under torsional forces imposed on the torque gate by the steered mechanism, the spring locks against the stationary surface so as prevent the drive surface of the locking driver from driving the release driver to rotate, thereby preventing torque transfer to the speed changer and the steering mechanism from the steered mechanism.
If the stationary surface is the inner peripheral surface of a clutch housing, a diameter of the housing may be adjustable to maintain a desired positional relationship between the coil of the spring and said inner peripheral surface of the housing.
The steering system is usable in virtually any off-highway application, including marine applications such as boats and land-based applications such as agricultural vehicles. In addition, some applications may have one or the other of the speed changer and torque gate but not both.
An improved method of transmitting torque to a steered mechanism such as a rudder or a steered wheel from a steering mechanism such as a steering wheel is also disclosed.